Heat-resistant cast iron



Patented Dec. 11, 1945 HEAT-RESISTANT CAST IRON Daniel E. Krause, Columbus, Ohio, assignor, by mesne assignments, to Gray Iron Research Institute, Inc., Columbus, hio,.a corporation 7 of Ohio No Drawing. Application August 6, 1943,

Serial No. 497,681

. s claims. ((21. 75-123) This invention relates to a heat-resistant cast iron, and it is concerned particularly with a composition which is-resis tant to' growth and to scaling at elevated temperatures.

A greatmany alloys have been proposed in the prior are for use at elevated temperatures. Ordinary gray cast,irons are satisfactory for service at temperatures up to approximately 800 to 850 F., but these materials fail rapidly at higher temperatures. At temperatures in excess of about 1200 or 1300 F., such irons are subject to severe losses by oxidation and scaling and are also subject to permanent growth which results in increased brittleness and decreased.

strength.

In all probability, the severity of scaling, p rmanent growth, and other deleterious eiiects of high temperatures is primarily a result of the presence of the graphite in the gray cast iron, which gives the iron an open structure. When such irons are heated in an oxidizing atmosphere, the hot gases attack the surface and penetrate into the iron along small cracks that are opened up as a result of the difiference in thermal expansion between the iron matrix and the graphite. These gases then react with the iron in the cracks, producing oxides having agreater volume than the iron from which they were derived. This results in a permanent increase in the volume of the iron member, which, in some cases, may be as great as 50 per cent.

The phenomena of growth and scaling,- however, are not limited to gray cast irons which contain graphite flakes. Although steel-s are not subject to marked growth, this deleterious action occurs in cast irons which, because of a suitable composition and cooling rate, have all of their carbon originally in the combined form. It is possible that in these irons growth results from at least partial malleabilization at the temperature of service. Fully malleabilized irons are also subject to rapid growth. A When severe permanent growth occurs in cast iron, the iron becomes unserviceable because of the loss of strength and increased brittleness and because of the fact that dimensional tolerances are exceeded in the case of close fitting parts.

In the prior art, it has become general practice to employ steels of the stainless variety, containing relatively large amounts of chromium or nickel or both,'in applications where strength and resistance to oxidation are required at temperatures in excess of around .1800 F. For service at intermediate temperatures in the general range of from 1000 to 1800 F., certain alloyed cast irons have been used. For exacting service in this temperature range, austenitic cast irons,

containing a high nickel content as well as copper, chromium and silicon, have been recom- .mended. These irons, however, because of their high alloy content, are quite expensive. ,Another iron that has good heat resistance con tains from 5 to 7 percent of silicon. Such an iron, however, has extremely. poor mechanical properties.

For somewhat-less exacting services in the intermediate temperature range, it is customary to employ chromium to improve the heat resistance of cast iron. In some instances, nickel, copper, or molybdenum, or their combinations, may also be added to enhance mildly the heat resistance conferred by the chromium, as well as. to improve the-casting and mechanical properties of the iron. Such chromium additions, in quantities up to one or more per cent, are effective in conferring growth and scaling resistance tocast iron andare, therefore, contained in the. bulk of all gray iron castings which must resist the deleterious influences of exposure at elevated temperatures.

be desirable to increase the heat resisting propert es of cast iron still further without the use of excessive amounts of alloying elements.

It is therefore one object of my invention to provide an improved heat-resistant cast iron.

Another object of this invention is to provide a cast iron having growth and high temperature,

' scale resistance in wh ch the-heat resistance conferred by chromium is markedly enhanced.

A further object of my invention is to provide a method of producing an iron of relatively low alloy content having superior resistance to growth and scaling at elevated temperatures.

Other and further objects of this invention will become apparent from the following description and the appended claims.

The beneficial results of my invention, whereby improved growth and high temperature scaling resistance is obtained in a cast iron of relatively low alloy content, are achieved by moderate ad.- ditions of boron to the iron. The amount of boron to be added is not critical within the hereinafter disclosed limits, enhanced heat resistance being obtained from boron additions varying from mere traces to more than 1.0 per cent. In general, however, I prefer to acid from 0.05 to 0.5 per cent.

Boron, by itself, is effective in markedly improving both the resistance to growth and the resistance to scaling; however, if desired, the boron may be added to an iron in which chromium are present, thereby further improvingthe heat-resistant properties.

As an example of the. eflicacy of boron in improving theheat resistance of cast iron, data on a series of growth tests are presented in Table I.

In these tests, specimens 1 inch in diameter and 4 inches long were subjected to twelve heating cycles; each cycle consisted of heating rapidly to However, in spite of the beneficial effects produced by chromium additionait would 1500 F. in an electric muflie furnace, holding for four hours at 1500 F., and cooling rapidly to room temperature. The initial length of the specimen and its length after each cycle were determined, from which the total accumulated growths exhibited by the' specimens wereobtained.

Tam: I

Efiect of boron on the growth of cast iron Composition ggfig g Cast iron I B or After After After 1st cycle 6th cycle 12th cycle Per cent Per 'cent A 0. 0. 0 13 35 A- 0. l7 0. 0 10 22 53 B 0. 0 0. 0 11 57 B 0. 06 1 0. 0 13 47 103 B- 0.12 0. 0 10 33 72 B. 0. O 0. l4 6 43 93 C. 0.0 v 0.16 14 30 54 C 0.07 0. 16 14 27 46 C 0. l6 0. 16 14 23 35 C 0. 0 0. 41 9 31 Composition of base iron: A-Carbon, 3.21%; silicon, 2.08%;

manganese, 0.73%. I3Carbon, 3.33%; silicon, 2.02%; manganese,

0.66%. C-Carbon, 3.40%; silicon, 2.53%; manganese, 0.71%.

As shown by the data in Table I, as'little as 0.06 per cent of boron is effective in markedly silicon, decreased the growth after 12 cycles of heating to-1500 F. by more than 50 per cent These data also show that relatively small boron additions are more effective than equivalent additions of chromium. The ability of boron additions to efiect an improvement in the heatresistant qualities of chromium-bearing cast irons is also clearly shown by the data of Table I. In general, the relative efiicacy of boron in reducing the growth of gray cast iron tends to increase as the number of high temperature cycles to which the iron is subjected is increased.

To determine the influence of boron on scaling resistance, the growthspecimens used to obtain the data recorded in Table I were weighed initially and after cleaning at the end of the twelfth cycle of heating to 1500? F. The weight losses obtained as a result of scalin are recorded in Table II. These data clearly show that boron additions are quite eiiective in decreasing the scaling of cast iron at elevated temperatures. Again, the ability of boron to increase the heat-resistant properties of chromium-bearing cast irons is 1 Composition of base irons same as shown in Table I.

As a further illustration of the influenc of boron on scaling resistance, specimens of cast iron of varying boron contentwere held for 24 hours at 1500 F. in air and the loss in weight by scaling determined. The specimens were approximately 1% inches long by inch square and gave the results recorded in Table III.

TABLE .III

iron

Total Weight loss after re- Cast 1 moving scale Gram Composition of bme iron: D-Carbon, 3.06%; silicon, 2.08%; man anese, 0.58%. E-Darbon, 291%; silicon, 2.00%; manganese,

If-Carbon, 3.24%; silicon, 2.21% manganese. 0.44%. The reduction in growth effected by boron additions in accordance with my invention is, therefore, apparent. It is also apparent that minor boron additions are effective and that the efficacy of the treatment increase as the boron content is increased. Furthermore, I have discovered that boron in small quantities is more effective in. reducing growth than is the same percentage of chromium. The effect of mixtures of boron and chromium shows that the two elements operate more or less additively in reducing growth. I have also found that boron-treated irons are superior in settling resistance and that boron in small quantities is more efiective in reducing scaling than is an equal percentage of chromium.

Boron additions to cast iron increase the chilling tendencies and form a constituent of a network pattern in the microstructure. This constituent resists to a marked degree decomposition at elevated temperatures, while imparting to the iron improved resistance to scaling. The stability of.this boron constituent, with its apparent oxidation-resistant qualities, appears to play a part in accounting for the inhibiting efiect of boron on the growth of cast ironat elevate temperatures.

The cast iron of my invention constitutes a definite improvement over the low alloy irons customarily employed to resist scaling and growth at elevated temperatures. It is adapted to applications where service conditions are not extraordinarily severe, that is where the temperatures do not exceed approximately 1800 F., and it is readily prepared from metal melted in the cupola,

the air furnace, the electric furnace, or any other suitable melting unit. The boron may be added during tapping or to the metal in the ladle when it cannot be added conveniently during melting or the preparation of the metal in the furnace. Boron, in amounts up to about 1.0 per cent, does not adversely affect the casting properties of the iron. Y

The form in which the boron is added is not limited within the scope of my invention. It may be added as a relatively pure material or in combination with iron as ferroboron. It also may be added as a complex addition agent in conjunction with such elements as chromium, titanium, zirconium, calcium, and vanadium.

Although any low alloy cast iron may be used as the base for the improved cast iron of my invention, a typical base composition may con tain carbon from about 1.7 to 3.7 per cent, silicon from about 0.50 to 3.50 per cent, manganese from about 0.20 to about 2.0 per cent, phosphorus from about 0.03 to 1.0 per cent, and sulfur from 0.02

to 0.15 per cent. In addition, other elements mayminor amounts of other alloying elements, the

balance of the composition, at least 80 per cent by weight, being substantially iron. To such a composition, I achieve my desirable results by adding up to approximately 1.0 per cent boron, together with such elements as may be associated with it as an addition agent.

'A typical cast iron of my invention may be gray, white, or of a character which subsequently can be annealed or malleablized to obtain nodular graphite and ductility. The balance of the chemical constituents in the iron will be determined by the properties desired, the nature of the service, and the temperature to which the iron is exposed. If, for example, theiron is to remain gray as cast, it must contain a proper balance of graphite-producing elements so that the structure formed comprises a substantial percentage of graphitic carbon. The control of the structure is largely dependent on the silicon and carbon contents, although strong carbidetorming elements as well as boron, have a profound eflect.

v In general. I have. found that to maintain a gray structure with the usual ratio of carbon and silicon and the usual rates of cooling, the boron content should be kept below about 0.3 per cent. White cast iron and cast iron which is subsequently malleablized to provide nodular graphite may contain higher percentages of boron.

In accordance with my invention, the improved heat-resistant cast iron may be heat treated to effect further improvement in its properties. While specific examples have been given in the foregoing specification, these examples are given merely to illustrate the-principles of my invention, and the invention is not limited to or by such examples.

Having thus described my invention, what I claim is:

l. A machinable, heat resistant, gray cast iron containing from 0.05 to 0.30 per cent of boron.

2. A material having the characteristics required for utilization in heat-resisting applications at temperatures below about 1800 F. said material comprising a machinable gray cast iron containing from about 0.05 to about 0.50 per cent of boron.

3. A machinable heat resistant, gray cast iron containing from 0.05 to 0.50 per cent of boron.

DANIEL El KRAUSE. 

